In a communication system, transceivers are electronic devices that include receivers that receive incoming signals and transmitters that transmit outgoing signals, and are very well known.
Certain types of digital receivers have the ability to receive incoming signals that are transmitted with different modulation schemes, also known as multi-rate modulation schemes. Thus, a single receiver can, at different times, receive signals that have been modulated in, for instance, a binary phase shift key (BPSK) modulation format or a quadrature amplitude modulation (QAM) format. Generally, it is advantageous to use a more complex modulation scheme if possible, since more information can be communicated for a given bandwidth using a modulation scheme that is more complex. But which modulation scheme to use at different times can depend on various criteria. For instance, when a communication channel is good, the transmitter may try to transmit at a high data rate with a complex modulation, such as 64 QAM, but when the channel is poor, a lower data rate with a less complex modulation, such as BPSK, may be used. When switching between modulation formats, the transmitter will commonly include a header that is modulated using the lower data rate modulation scheme. This header will typically also include an indication of the modulation used for the rest of the packet. When received at the receiver, this lowest common denominator signal at the lower data rate can, with assurances, be detected and demodulated.
Related to the type of modulation scheme that is employed are the concepts of gain and signal to noise ratio (SNR). In the receive path, gain is the term used to refer to the amplification that is applied to the incoming signal. And the SNR refers to the ratio of the incoming signal to the noise that is present in the receive path of the circuit due to components therein, such as amplifiers and mixers.
Most receivers include programmable gain in the receive path, since during the usage of the receivers, changes in the channel, circuit characteristics and the like will cause the need for a different gain at different times. Accordingly, the programmable gain is used to optimize the scaling of the received signals within the dynamic range of the receiver. For example, if the gain is set too low, noise from the various amplifiers and mixers in the receive chain will be significant compared to the size of the desired signal, and will thereby degrade performance. On the other hand, if the gain is set too high, the desired signal, or undesired signals in nearby frequency channels will cause the amplifiers and mixers to clip or saturate. The resulting non-linear behavior of these active circuits would degrade the performance of the communication system.
It is, therefore, important to optimize the gain, and thus the signal scaling, at all times in the receive chain. The optimum signal sizing is just large enough to insure that any circuit noise remains small enough relative to the signal size that successful communication can be maintained. By keeping the gain at this lowest allowable level, potentially interfering signals are allowed to be as large as possible without causing the active circuits to saturate.
In order to optimize the gain, and thus the signal scaling that is appropriate for a given packet, the minimum acceptable SNR can be calculated, simulated, or found from experimental measurements, and then used to determine the gain. And the required SNR depends on the complexity of the signal being transmitted. For example, modulation schemes that use more complex constellations, such as 64-QAM, require higher SNR.
When transmitting signals when using a multi-carrier modulation format, such as Orthogonal Frequency Division Multiplexing (OFDM) and Discrete Multi-Tone (DMT), there are included intervals when no information is being transmitted, which intervals are expressed as guard times or guard intervals. A guard time exists between each of the transmitted symbols, and is long enough to span the time of the multi-path echoes that will occur in the channel. In the receiver, these guard times are intentionally ignored, so that the multi-path echoes do not corrupt the decoding of the data. As will be described hereinafter, these guard periods present an opportunity to adjust the gain in the receiver, without causing data loss due to the temporary disruptions that occur when the gain is changed. There are also other times when it may be appropriate to adjust the gain in the receiver, such as when padding bits are being transmitted. Because the modulation type may change in the middle of a packet (at least after the header), scaling the signal to its optimum level is challenging. Since the receiver does not know the modulation format, and thus the data rate of the body of the packet, immediately, it cannot know the optimum scaling to use at the beginning of the packet. Therefore, the receiver must be conservative and size the signal large enough so that even if the most complex modulation is used later in the packet, sufficient SNR will exist so that it is received correctly. While this signal sizing will prove correct if the packet really does contain data modulated in the most complex way, if the packet contains data that is modulated in a less complex way, then that signal sizing will have been larger than necessary, and sacrifice potential ability to withstand interference. In setting the signal size, consideration must be given to the worst-case power back-off due to the blocker power.
An example of multi-modulation format signals are the signals associated with the IEEE 802.11a standard or Hiperlan II standard, which each allow for high-speed local area network communications in the 5 GHz communications band. The signal in the 802.11a standard is allocated into one of twelve different 20 MHz channels. Each of the eight channels is divided into 52 different sub-channels or carriers, of which 48 carriers are able to transmit the signal and 4 of the carriers are used to transmit pilot tones. During transmission, the signal is spread onto each of the 48 carriers associated with the channel according to the modulation scheme used, and, upon receipt, is despread and demodulated to regain the originally transmitted signal.
FIG. 1 illustrates the beginning portion of a packet for such an OFDM signal 100, which includes ten short training symbols t1–t10, which are identical to each other and used for signal detection, an initial automatic gain control adjustment, diversity selection, coarse frequency offset estimation and timing synchronization. Two long training symbols T1 and T2 that are also identical to each other are typically used for channel and fine frequency offset estimation. Thereafter exists the SIGNAL symbol, which corresponds to the header referred to above, that contains information indicating the data rate at which the following data, illustrated as Data 1, Data 2 , . . . , for the remainder of the packet, will be transmitted. In the 802.11a standard, for each different data rate there is a different modulation scheme, which results in a one-to-one correspondence between the data rate and modulation scheme.
FIG. 2 illustrates a functional block diagram of a conventional receiver 200 that can be used to receive the signal 100 illustrated in FIG. 1. As illustrated, the receiver block includes a low noise amplifier 210, which provides an initial amplification to the received signal. Mixer 212 and automatic gain control amplifier and radio frequency level detect circuit 214 then downconvert the RF signal, typically through both an IF and then to baseband, and amplify the signal at each stage based upon a determined appropriate gain, in part based upon estimates of the in-band power of the signal. Typically the various analog gain stages are automatically controlled through digital signals, and many power estimation algorithms exist that can provide these gain settings. An IQ detector 216, in conjunction with an AFC clock recovery circuit 218, will detect the I and Q phases of the baseband signal. Gain control circuit 220 detects the magnitude of the I and Q phases of the detected baseband signal output from the IQ detector 216 during the short training symbol sequence thereof and uses the detected magnitude to adjust the gain of the gain control amplifier 214. A symbol timing circuit 222 also receives the output of the IQ detector 216, and determines those intervals during which an actual symbol exists, rather than a guard interval, and provides a timing output to the fast Fourier transform (FFT) circuit 224, which also receives the output of the IQ detector 216, and causes FFT 224 to be gated in time to receive the signal data, rather than noise caused by interference that will exist during a guard interval. The output of the FFT 224 is provided to a channel estimation and pilot phase tracking circuit 226, and a channel correction circuit 228.
The channel estimator within the channel estimation and pilot phase tracking circuit 226 obtains a channel estimate during the long training symbol sequence, and provides that channel estimate to the channel correction circuit 228. That channel estimate is then used by the channel correction circuit 228 to compensate for the determined channel characteristics for the rest of the packet. And, if included in circuit 226, a pilot phase tracker will adjust the channel estimate based upon channel information obtained by tracking pilot tones during the transmission of the rest of the packet. The channel corrected signal is then input to the de-mapping and de-interleaving circuit 230, and the forward error correction (FEC) decoder 232, typically a Viterbi decoder. The circuit 230 and the decoder 232 together decode the signal, in a conventional manner. In such a conventional receiver 200, as illustrated, the information in the packet, including the information contained in the SIGNAL symbol, is not available until the FEC decoder 232 has completed its operation, which will not occur until after quite a bit of data, as represented by Data 1, Data 2 . . . , has already been received and amplified by the gain control amplifier 214.
In operation of a conventional receiver of this type, the gain used by the automatic gain control amplifier of circuit 214 is initially determined during the initial short symbol training sequence, and then kept constant for the remainder of the packet. Thus, since the gain is held constant thereafter, it must be maintained at a level that allows the sizing of the received symbols to be large enough such that even if the most complex modulation is used in later symbols in the packet, sufficient SNR will exist so that it is received correctly, as described above.
Other types of receivers operate by continuously varying the gain to insure that whatever signals are coming in do not overload the receiver. This has at least two disadvantages in a packet-based communication system. First, the undesired signals may come and go abruptly in a packet-based system, and by the time they are detected, the desired packet may already be ruined. Second, since it is necessary to change the gain almost immediately to prevent such overload, the gain might need to be changed in the middle of a symbol. But changing the gain in the middle of a symbol can cause data to be ruined due to the change in signal magnitude and/or phase.
Accordingly, another method to control the timing of gain changes is needed, particularly when the modulation scheme changes within a given packet.